The overall aim of this grant is to study the immunopathogenesis of multiple sclerosis (MS) and to define immunoregulatory pathways that may control disease. In addition, we will use the opportunity presented by ongoing treatment of MS patients with IFN-beta and a planned clinical trial of a humanized monoclonal antibody to CD40L in MS to evaluate basic mechanisms of these therapeutic interventions in this disorder. Based on clinical evidence as well as data we have generated in EAE we hypothesize that there are four principal events in the development of MS: (1) genes including MHC alleles predisposing to MS establish a T-cell repertoire capable of recognizing self myelin related peptides intrinsic to the autoimmune process of MS: (2) myelin specific CD4+ T cell clones previous tolerant to autoantigen become activated and expand to change the T cell repertoire to reflect autoreactive CD4 T cells; (3) regulatory mechanisms, including the activation of TH1 and TH2 CD4+ T cell subsets as well as those involving regulatory CD8 T-cells fail, through processes, such as clonal deletion or changes in the cytokine milieu and (4) potentially myelin reactive TH1 CD4+ T-cells migrate to the CNS and induce tissue damage and disease. There will be two interactive aspects of this project. The first aspect will consist of three specific aims designed to study basic aspects of the immunopathogenesis and immunoregulation of human MS. These aims will: (1) determine if regulatory CD8+ T cells which specifically down-regulate MBP specific CD4+ T cells exist in the PBL or CSF of MS patients. (2) determine if the MBP specific CD4+ T cell TCRVB or functional repertoires are different at different disease stages and are modified by CD8+ T cells: (3) identify antigen activated CD4+ T cell clones in the CSF or MS patients, immortalize these clones and use libraries of T cell clones to determine antigen specificity. The second aspect of studies will consist of several specific aims directly concerned with the investigation of immunoregulatory mechanisms and TCRVB functional repertoires in patients being treated with IFN-beta or anti-CD40L.